1. Field of the Invention
The subject invention relates to a spark plug for an internal combustion engine, furnace, or the like wherein the spark plug includes at least one electrode having a wear-resistant sleeve welded thereto for enhanced durability and longevity.
2. Related Art
Within the field of spark plugs, there exists a continuing need to improve the erosion and corrosion resistance and reduce the sparking voltage needed to produce the spark in the gap between center and ground electrodes. To this end, various designs have been proposed using noble and/or precious metal firing tips applied to standard metal electrodes. Typically, the firing tip is pre-formed as a pad, rivet or wire which is later welded onto the end of either the center electrode, the ground electrode, or both.
Platinum and iridium alloys are two of the noble metals commonly used for these firing tips. Platinum-tungsten alloys have also been used, along with platinum-rhodium alloys and platinum-iridium-tungsten alloys. Other metals and/or alloys are also possible.
While these and various other noble metal systems typically provide acceptable spark plug performance, particularly with respect to controlling the spark performance and providing spark erosion and chemical corrosion protection, current spark plugs utilizing noble metal tips have well-known performance limitations associated with the relatively small sparking surfaces and with the methods which are used to attach the noble metal components, including various forms of welding. In particular, cyclic thermal stresses in the operating environment, such as those resulting from the mismatch in the thermal expansion coefficients between the electrode tip and the dissimilar base electrode, can decrease service life. Typically, the electrode tip will be fabricated from noble metals and the noble metal alloys mentioned above, whereas the base electrode will be made from nickel, nickel alloy, nickel clad copper, or other commonly used metal. The result of these mismatched thermal coefficients is cracking, thermal fatigue, and various other interaction phenomena that can result in the failure of the welds and, ultimately, of the spark plug itself.
The condition is particularly significant in the field of industrial power generation, wherein a spark plug may be operated for extended durations at a specified setting. In these types of applications, which are cited merely by way of example, it is desirable to very precisely tune the engine and its fuel supply, together with the ignition system, to obtain the highest possible efficiencies and fuel economies. Erosion and corrosion of the center and ground electrodes can have a profound effect on the efficiency and performance characteristics of such an engine. Accordingly, there is a great need in this field to provide a spark plug having improved erosion and corrosion resistance of the sparking surfaces and related components.
The prior art has long considered this situation and proposed numerous configurations within which to deploy noble metal components in the spark gap. For example, U.S. Pat. No. 4,904,216 to Kagawa discloses a spark plug having a center electrode fitted with a tubular precious metal sleeve that is attached by resistance welding and then afterward drawn and extruded to a final shape. In another example, U.S. Pat. No. 5,557,158 to Kanao et al., discloses a spark plug including a center electrode that is fitted with a tubular precious metal sleeve. The sleeve is captured on a tenon end and then fixed in position via a cap. In yet another example, U.S. Pat. No. 6,064,144 to Knoll et al., discloses a spark plug wherein a tubular sleeve is fitted to a tenon on the center electrode and retained in position by a compressing cinch. This is followed by a welding or soldering operation.
Accordingly, it is highly desirable to develop a spark plug having a noble metal firing tip in the form of a sleeve or other configuration applied to the sparking end of the center electrode. However, the prior art attempts have failed to account for potential failure mechanisms associated with the attachment of dissimilar materials to one another over a length, and which materials are subjected to intense thermal cycling. Accordingly, there is a need to develop methods of making spark plugs having improved structures so as to improve spark plug performance and reliability, while also sustaining component integrity in extremely harsh operating environments.